Carbon dioxide release from fossil-fuel burning is significant enough that we may soon experience perceptible changes in climate with important human consequences. Man's activities involving deforestation and agriculture have undoubtedly also affected atmospheric CO(,2), although quantitative, and even qualitative, net effects of these processes are incompletely understood relative to fossil-fuel production. An accurate reconstruction of past ('13)C/('12)C ratios of atmospheric CO(,2) may provide key constraints on the historical activity of the biosphere as CO(,2) source or sink. Tree rings appear to be a repository of this information but there is much noise in the collection of previous reconstructions, presumably associated with site selection, radial variability, choice of representative wood chemical constituent, and subtle effects of climate on fractionation. This study attempts to avoid these pitfalls and develop a 50-yr (delta)('13)C(,ATM) record from juniper trees (genus Juniperus), in fact, by taking advantage of the influence of climate on fractionation. Trees were harvested from suitable sites in close proximity to weather stations with monthly records of temperature and precipitation. Ring material was then separated from each of the sections in 5-yr intervals from 1930 to 1979 around their full circumference, and cellulose was extracted from the wood. After measuring (delta)('13)C of the cellulose by standard mass-spectrometric techniques, a variety of (delta)('13)C vs. climate functions were examined for each interval. The most useful relationships for at most 7 of the 10 sites were (delta)('13)C with December temperature or precipitation, because the coefficients were nearly constant from one interval to the next (averaging -0.27('o)/oo(DEGREES)C('-1) for temperature and -0.04('o)/oo mm('-1) for precipitation) and the intercepts differed. Local pollution effects are believed responsible for the three anomalous sites. The separation of these regression lines of different intervals is interpreted as the response of the trees to the changing (delta)('13)C of atmospheric CO(,2) so that (delta)('13)C(,ATM) curves are constructed from this spacing. The shape of the best-fit reconstruction suggests the biosphere has acted as CO(,2) source to about 1965 and may now be a net sink. Although these conclusions are limited by certain assumptions and statistical restrictions, evidence from the recent scientific literature tends to support the increasing role of the biosphere as an important carbon sink.